1. Field of the Invention
The present invention relates generally to the field of lithotripters used for noninvasive fragmentation of concrements in living beings. More specifically, the present invention discloses an improved spark gap electrode for lithotripters in which the discharge electrode tip can be readily removed and a replacement tip installed with precise mechanical positioning.
2. Statement of the Problem
Conventional extra-corporeal lithotripters utilize shock waves to noninvasively fragment concrements within a patient (e.g., renal and ureteral calculi located primarily in the upper urinary tract). Conventional lithotripters comprise a truncated ellipsoidal reflector. The reflector is filled with a liquid, e.g., water, that couples the interior of the reflector to the patient's body, and through the tissues of the body to the concrement to be destroyed. The reflector is provided with a spark gap at the first focal point of the ellipsoid. The concrement is positioned coincident with the second focal point of the reflector by means of x-ray or ultrasound imaging. An electrical discharge across the electrodes of the spark gap causes a certain amount of water to be flashed into steam, accompanied by generation of a shock wave. This shock wave is focused by the ellipsoidal reflector on the second focal point, and hence on the concrement. A rapid succession of such sparks is sufficient to substantially disintegrate any concrement.
However, the rapid and frequent discharge of energy across the electrode tips of the spark gap also causes rapid erosion and deterioration of the electrode tips. Spark gap electrodes typically last no longer than one treatment and often must be changed during treatment. The conventional solution to this problem is to frequently replace the entire electrode assembly. For example, a side cross-sectional view of the prior art spark gap electrode assembly produced by Medstone is shown in FIG. 5. FIG. 6 is a side cross-sectional view of the prior art spark gap electrode assembly produced by Dornier. The Dornier device does not provide a means for replacing the electrode tips. The Medstone device can be rebuilt with substantial difficulty, but replacement of the electrode tips is relatively complicated, cumbersome, and requires manual calibration of the gap between the electrode tips.
Other prior art spark gap electrode assemblies include means for periodically adjusting the spacing between the electrode tips to compensate for burn-off during use. This approach requires a complicated mechanical arrangement to allow the electrode tips to be regapped in place. Such devices are relatively expensive. The regapping process is time consuming, subject to error, and can result in inferior mechanical and/or electrical properties for the device.
In particular, a number of spark gap electrode assemblies have been invented in the past, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Nowacki et al. 5,047,685 Sept. 10, 1991 Nowacki et al. 4,905,674 March 6, 1990 Pimiskern 4,905,673 March 6, 1990 Muller et al. 4,608,983 Sept. 2, 1986 Hepp et al. 3,970,076 July 20, 1976 Hoff et al. 3,942,531 March 9, 1976 Poston 3,728,671 Apr. 17, 1973 Rieber 2,559,227 July 3, 1951 ______________________________________
The patents of Nowacki et al. disclose two examples of electrode structures for a lithotripter.
Pimiskern discloses an arc discharge device for shock wave generation wherein the two electrodes 4, 6 have flattened tips of different diameters.
Muller et al. disclose a shock wave generator for breaking up concrements (e.g., kidney stones, gallstones, or bladder stones) within a living body. The device has an ellipsoidal reflection chamber having two focal points, one to be aligned with the concrement to be destroyed and the other located in a spark discharge path between two electrodes. One of the electrodes 36 is a point-like extension of an inner conductor. The outer conductor carries several flat arch-shaped holders to position the other electrode. A threaded mounting arrangement allows the two electrodes to be moved toward and away from each other to compensate for burn-off of the electrodes during use. Two mounts 64 and 66 hold the electrodes in a chuck-like fashion to facilitate easy replacement of the electrodes. The small size of the mounts would require tiny electrodes that are subject to rapid burn-off during use. In addition, fine adjustment and initial calibration of the electrode spacing would be required.
Hepp et al. disclose an apparatus for heart stimulation using pressure waves produced by a spark discharge within an elliptical coupling chamber.
Hoff et al. disclose another example of a shock wave generator for breaking up concrements within a living body. This apparatus uses a spark gap at one focal point within an elliptical waveguide.
Poston discloses a multiple-electrode directional acoustic source. Concentric electrode pairs of opposite polarity improve the efficiency of a spark gap acoustic source for marine seismic profiling. One electrode of each pair is tubular. The other electrode is rod-like and positioned axially within the tubular electrode.
Rieber discloses another example of a shock wave generator using a spark gap at one focal point within an ellipsoid shell.
3. Solution to the Problem
None of the prior art references uncovered in the search show a spark gap electrode for lithotripters that allows the electrode tips to be easily and quickly removed and replacement tips installed with precise mechanical positioning. Manual calibration or adjustment of the gap between the electrode tips is not required.